Aircraft are typically required to fly in weather conditions in which the airframe structure may be subjected to lightning strikes. Such lightning strikes often attach to the aircraft at a fastener of a fastened joint, or are conducted via aircraft structure to such a fastener. It is necessary to predict and control the effects of such lightning strikes, particularly at fuel tank joints, in order to prevent fuel ignition.
FIG. 1 is a side view of part of a fastener assembly passing through a panel 1, which may be a composite or metallic panel. The assembly comprises a fastener comprising an externally threaded bolt 2, an internally threaded nut 3, and a washer 4 (the fastener may alternatively comprise any other known fastener type, such as a rivet or swage fastener). In the event of a lightning strike hitting the panel 1 and current flowing through the fastener, sparking at the joint (e.g. at the interface between the panel 1 and the fastener) may cause localised pressure build up and ejection of a shower of sparks from the fastener. The pressure build up may also cause gases, plasma and hot particles to be released from the joint.
Typical locations of such out-gassing are indicated by reference 5 in FIG. 1. The panel 1 may provide a fuel tank boundary and the fastener may therefore be immersed in fuel or fuel vapour. A lightning strike at the fastener may thus provide sparking and hot gas ignition sources which could cause ignition of the fuel vapour. Other electrical current threats, such as threats from electrical equipment on the aircraft, may also provide such ignition sources.
In some cases the pressure build up may be sufficiently high to cause structural damage to the panel 1. Where the panel is made from a composite material, the pressure may cause delamination as the gas, plasma and/or particles try to escape from the joint.
Previous testing methods used in the aerospace industry to detect fuel ignition sources during simulated lightning strike tests are based upon photography and/or flammable gas techniques, as defined in EUROCAE ED-105, Section 7.7 and equivalent SAE and MIL-STD standard documents. Such techniques provide a discrete pass/fail result, without any quantitative information about the characteristics of the out-gassing event and associated margins. There are no known methods or devices for measuring physical properties of the gases, plasma and/or particles released during an out-gassing event.
In fact, there are still several unknowns about the physical mechanisms at work during an out-gassing event. There is therefore a need for a device and methods for creating improved data about such out-gassing events. There is also a need to better understand the influence of design features of the joint on the degree of out-gassing.